Various improvements are tried in providing an optical pickup of compact size and light weight. As one example, a pickup where a pickup holder having an objective for an optical system is suspended from a suspension base by a twin-shaft spring in the form of a cantilever has been developed.
FIG. 7 shows a conventional optical pickup. The optical pickup comprises a pickup body 20 for mounting an optical system and a yoke base 10 provided with an actuator and mounted on the pickup body 20.
The yoke base 10 is made of magnetic material and comprises a base plate 11 and a pair of side plates 13. A pair of yokes 15 are secured to the base plate 11. A pair of magnets 14 are, secured to the side plates 13 adjacent the yokes 15. A magnetic circuit is formed by the magnets 14 and the yokes 15. A suspension base 12 is provided on a flange portion 13a of one of the side plates 13. A holder 17 as a base of a pickup head is suspended from the yoke base 10 by conductive four spring wires 18 in the form of a cantilever. The holder 17 has an objective 16 for the optical system and a pair of coil substrates (not shown) for tracking and focusing. The holder 17 is disposed between the yokes 15 so as to oppose the coil substrates to the yokes, respectively.
The pickup body 20 is made of plastic or metal such as aluminum or zinc formed by projection molding or machining. The body 20 has recessed portions 21, 22 and 23 in which various elements for the optical system are mounted. A semiconductor laser device 24 is mounted in the portion 21. A half mirror 25 and an optical electronic integrated circuit (OEIC) 26 are mounted in the portion 23. A reflecting mirror 27 is mounted in the portion 22.
The yoke base 10 is secured to the pickup body 20 by screws 30 engaged with holes 19 of the base 10 and holes 20a formed in the pickup body 20.
In operation, a laser beam emitted from the semiconductor laser device 24 is reflected on the half mirror 25 to the reflecting mirror 27 passing through a collimator lens (not shown) where the beam is shaped into a parallel beam. The beam is reflected on the reflecting mirror 27 to the objective 16 and focused on an optical disc (not shown) by the objective 16.
The reflected beam from the disc passes in reverse and is reflected on the half mirror 25 to the circuit 26 and focused thereon by a condenser lens (not shown).
Since the yoke base 10 made of magnetic material is secured to the pickup body 20 made of another material such as plastic, aluminum or zinc, both the members undergo different amounts of thermal expansion because of the different coefficients of thermal expansion. As a result, both members curve at the engaging surface. Accordingly, the relative positions among the laser device 24, half mirror 25 and reflecting mirror 27 of the body 20 are changed, causing the optical axis of the laser beam to deflect from the designed optical axis.
Accordingly, the reliability of the pickup is deteriorated.